FIG. 1 is a flowsheet of a known method of extracting cobalt from a nickel sulphate solution with a direct addition of ammonia to a cobalt solvent extraction phase. A problem with this method is that the concentrated nickel sulphate solution with the direct addition of ammonia results in the formation of insoluble nickel ammonium sulphate double salts. Australian patent No. 667539 in the name of Outokumpu sets out to avoid the formation of this double salt by a two stage process involving:    i) pre-neutralisation of a cationic extractant such as CYANEX 272 to form the ammonium salt; and    ii) pre-extraction or exchange of the CYANEX 272 ammonium salt with magnesium sulphate in an aqueous solution to form a CYANEX 272 magnesium salt which is contacted with an aqueous nickel sulphate solution in a solvent extraction circuit so as to extract nickel.
The applicants International patent application No. PCT/AU98/00457 avoids the relatively expensive two stage pre-equilibration of Outokumpu by adding chemically reactive magnesia, magnesium hydroxide, or magnesium carbonate to the cationic extractant without the pre-neutralisation step. However, if magnesia or magnesium pre-equilibrated extractant is used to avoid the formation of double salts this introduces magnesium ions which contaminate the final ammonium sulphate product.